Starting to hate my step kids.

Me and my partner have been together for 3 years and have a 5 month old daughter together. She's our rainbow baby, and we've been through hell together. We are so strong, and our relationship is very healthy. We recently got engaged. Back story: he has 2 kids from a previous toxic relationship, and they are 11 and 10 years old. They are with us 50% of the time, so it's quite often they are here. Their mum is toxic still and has successfully ruined my relationship with the kids. In the start, I had a good relationship with them both, but she got involved, keeps telling them bad things, keeps turning them against me for no reason. I hate it, and I now am starting to hate the kids because of their toxicity, too. It's always mum said this or my mum told me this and it's stuff like I'm a fat bitch, I'm horrible, I'm nothing to them so the kids don't need to do what I say and they can tell me to fuck off etc (they haven't done that, their dad would go crazy but it's what she tells them). I've never met this woman, and I don't want to. She's toxic. Now, the kids are becoming older, their attitudes stink, and they throw tantrums etc if stuff doesn't go their way. The eldest keeps stealing from our house and taking stuff to her mums, like books, creative things, toys etc and it's driving me nuts cos that stuff belongs here and then they have nothing to do here cos she's taking everything. We had to buy a lock for the kitchen cupboard cos she kept stealing food , full packets of biscuits etc. I'm a generally very calm person, patient and kind.. I was a teacher for 12 years and I've worked with kids and I'm not nieve to their behaviours etc. But I can't stop resenting them and thinking about how our relationship could have been if their mum wasn't involved and I just feel like it's a losses caused now after 3 years of this bullshit. And I'm tired. I don't want to try anymore. There's so many details I've left out but it'd just a constant battle. I want a simple life with myself, my partner and our daughter. I know its not possible and he would do anything for all his kids but I need to let it out. I need to say it.